Allow extensions to customize the search forms. This required some cleanup and refactoring to special:search and search engine.
Should be fully backwards compatible. Lightly tested, but only with MySQL search backend.
Introduces concept of search profiles, which replace long list of namespaces in the url.

Third -- whoever the audience who has to know what they are should be able to find that information. If we don't know what 'search profiles' are, we can't tell whether that's info that only people working on tweaking Special:Search have to worry about, or whether this is a concept that's being introduced to end-users that they'll have to select from and manage.